1. Field of the Invention
The present invention relates to automotive automatic transmissions of an engaging pressure electronically controlled type (which will be referred to EPEC hereinafter for ease of description) wherein the hydraulic pressure needed by each frictionally engaging element is directly controlled by an electronic control means, and more particularly to a hydraulic control system of such type automotive automatic transmissions.
2. Description of the Prior Art
For simplifying a hydraulic control circuit, reducing the number of parts employed and reducing the size of a control valve body, the EPEC type automatic transmissions have been proposed and put into practically use particularly in the field of motor vehicles.
One of the hydraulic control systems of automatic transmissions of such type is shown in Japanese Patent First Provisional Publication 7-77274.
As is seen from FIG. 10A of accompanying drawings, the hydraulic control system of the publication comprises a frictionally engaging element which, upon gear change, is engaged or disengaged by an element engaging hydraulic pressure xe2x80x9cPcxe2x80x9d, a solenoid valve which produces a solenoid pressure xe2x80x9cPsolxe2x80x9d upon receiving a solenoid""s output and a decompression control valve which produces or controls the element engaging hydraulic pressure xe2x80x9cPcxe2x80x9d in accordance with the solenoid pressure xe2x80x9cPsolxe2x80x9d and the element engaging hydraulic pressure xe2x80x9cPcxe2x80x9d. For producing the pressure xe2x80x9cPcxe2x80x9d which varies from 0 (zero) to the maximum (viz., a line pressure xe2x80x9cPLxe2x80x9d), the decompression control valve receives the line pressure xe2x80x9cPLxe2x80x9d while using both the pressures xe2x80x9cPsolxe2x80x9d and xe2x80x9cPcxe2x80x9d as signal pressures.
However, as is seen from the characteristic line (solid line) of the graph of FIG. 10B, due to the inevitable construction, the hydraulic control system is forced to have a larger gain (viz., a larger rate of change of the pressure xe2x80x9cPcxe2x80x9d to change of the pressure xe2x80x9cPsolxe2x80x9d), which tends to cause poor and unstable hydraulic controlling upon gear change.
One measure for eliminating such drawback is to lower the gain as is depicted by the characteristic line (viz., phantom line) of the graph. However, in this case, the element engaging pressure xe2x80x9cPcxe2x80x9d fails to reach the level of the line pressure xe2x80x9cPLxe2x80x9d even when the solenoid pressure xe2x80x9cPsolxe2x80x9d shows the maximum level. Accordingly, in this case, it is impossible to obtain a sufficient torque capacity needed for assuredly keeping the engaged condition of the frictionally engaging element after completion of gear change, and thus when the engine torque becomes large, slippage of the frictionally engaging element tends to occur.
That is, in order to avoid or at least minimize the slippage of the frictionally engaging element even when a marked torque is applied thereto in accordance with the driving condition of the associated vehicle, it is necessary to set the element engaging pressure xe2x80x9cPcxe2x80x9d at a higher level after completion of gear change. While, in order to reduce a shift shock at gear change, it is necessary to set the pressure xe2x80x9cPcxe2x80x9d at a relatively lower level suitable for the gear change. Even in inconsistent with each other, these two necessities are essential for providing a hydraulic control system of the automatic transmission with a satisfied performance.
It is therefore an object of the present invention to provide a hydraulic control system of an automotive automatic transmission, which is constructed by embodying the above-mentioned two necessities.
That is, in accordance with the present invention, there is provided a hydraulic control system of an automotive automatic transmission, that brings about both a satisfied controllability for a frictionally engaging element at a transitional period of gear change and a satisfied torque capacity for assuredly keeping the engaged condition of the engaging element after completion of gear change.
In the present invention, there is further provided a hydraulic control system of an automotive automatic transmission, that exhibits a stable and fine controlling performance against disturbance.
According to the present invention, there is provided a hydraulic control system for controlling an automatic transmission having a frictionally engaging element that is hydraulically actuated for assuming a certain gear position. The hydraulic control system comprises a first hydraulic section which produces a line pressure; a solenoid which produces a solenoid pressure; a decompression control valve which receives the line pressure to produce an output pressure while using both the solenoid pressure and the output pressure as signal pressures; a switching valve interposed between the decompression control valve and the frictionally engaging element, the switching valve having a first position wherein the output pressure is fed to the frictionally engaging element and a second position wherein the line pressure is fed to the frictionally engaging element; and a second hydraulic section that keeps the switching valve at the first position when a signal pressure applied thereto is lower than a predetermined switching pressure and turns the switching valve to the second position when the signal pressure becomes higher than the predetermined switching pressure.